The present invention relates to the magnetic resonance arts. It finds particular application in conjunction with the calibration of magnetic resonance imaging systems and will be described with particular reference thereto. It is to be appreciated, however, that the invention will find other applications in the diagnostic imaging arts.
In magnetic resonance imaging, gradient and RF radio frequency pulses are applied to dipoles in an imaging region to excite and manipulate resonance. The excited resonance is sampled and digitized and demodulated to create data lines for reconstruction into an image representation. Typically, sequence control circuitry controls the timing with which the gradient and radio frequency pulses are applied, the resultant resonance is sampled, digitized, and demodulated. It is normally assumed that the hardware which applies the gradient and radio frequency pulses and the hardware which samples, digitizes, and receives the resultant resonance all perform their tasks immediately upon being instructed or enabled by the sequence control processor, or at least that all of the hardware responds with the same speed.
To the contrary, the various hardware subsystems discussed above do not react with the same speed.
Moreover, the analogous hardware subsystem on one machine does not necessarily react with the same speed as the corresponding hardware subsystem on a like machine. These non-constant delays cause various pulses to be applied and data to be sampled and demodulated at other than the prescribed time. These small temporal shifts cause phase errors in the resultant resonance signals and the reconstructed data lines.
Magnetic resonance images suffer from numerous artifacts caused by phase and other errors. The above-discussed timing errors are only one of many sources of phase errors. One common technique for dealing with phase errors is to display a magnitude image. That is, the image data are complex numbers. The phase errors adversely affect the imaginary portion of complex numbers. Working with only the real portion (also known as the modulus) of the complex value enables many phase error problems to be eliminated. Phase errors are further corrected using various processing techniques both during the image reconstruction process and in post-processing operations. These techniques fail to address the temporal errors which are the cause of some phase errors.
Scanners are calibrated using various calibration techniques. For example, once an ideal imaging sequence is devised, the sequence is often manually and iteratively tweaked to optimize it. Typically, an operator will adjust gradient or pulse heights and other sequence parameters to optimize the resultant image. Once the sequence is optimized, the same optimized sequence is used on every like model scanner.
The present application overcomes the above-referenced problems and others.
In accordance with one aspect of the present invention, there is provided a method of calibrating a magnetic resonance imaging scanner which generates imaging sequences that include gradient pulses, RF pulses, and data sampling windows. An echo planar imaging sequence which includes an RF pulse followed by an oscillating read gradient and data sampling windows under positive and negative lobes of the oscillating read gradient is applied. The sampled data is analyzed for ghosting artifacts. A relative timing between the oscillating read gradient and the data sampling windows is adjusted.
In accordance with another aspect of the present invention, a further calibration method is provided. An oscillating read gradient and a coincident data sampling window are provided. An oscillating waveform controlling RF demodulation of the received MR signal is provided. Sampled data is collected both with and without the demodulation waveform, and pairs of images are generated. Ghosting is analyzed and compared between the pairs of corresponding images. Timing of the receive demodulation waveform is adjusted, relative to the oscillating readout gradient. When the pairs of images exhibit identical ghosting characteristics, the waveforms are considered to be adjusted so as to minimize timing errors and phase errors associated with the demodulation.
In accordance with yet another aspect of the present invention, a method of calibrating a magnetic resonance imaging scanner is provided. An oscillating read gradient having opposite polarity lobes of like size is applied. An RF pulse sequence which includes alternating positive and negative RF pulses of like tip angle is applied. After an even number of gradient pulse lobes and an even number of RF pulses, a rephasing gradient of one half the lobe size is applied and residual magnetization is read out in the presence of a sampling gradient pulse. A relative temporal starting point of the oscillating gradient and the RF sequence is adjusted. These steps are repeated until the phase error is minimized. An RF/gradient pulse combination is repeated. After completion of the pulse train, image data is collected. If the RF pulse and the gradient are not coincident, shading will result in the subsequent image. The relative timing of the RF/gradient combination is adjusted until the shading is removed. Data may be collected in the spaces between the RF/gradient pulse combinations as well as to achieve a similar result.
In accordance with yet another aspect of the present invention, a magnetic resonance imaging system is provided. A gradient hardware subsystem generates magnetic field gradients in an imaging region. A radio frequency transmission hardware subsystem generates radio frequency pulses in the imaging region. A data sampling and digitization hardware subsystem samples magnetic resonance signals from the imaging region during sampling windows. Demodulation means are provided to be applied to the received signal. A sequence controller sends control signals to the gradient hardware subsystem, the radio frequency transmission hardware subsystem, demodulation subsystem, and the data sampling hardware subsystem to control the application of gradient and RF pulses and sampling windows of a selected imaging sequence. A delay means adjusts relative temporal application of the control signals to the gradient hardware subsystem, the radio frequency transmission hardware subsystem, the demodulation subsystem, and the data sampling and digitization subsystem.
One advantage of the present invention is that it reduces inherent causes of phase error.
Another advantage of the present invention resides in the ability to calibrate individual machines with nominally identical hardware systems as well as those with different hardware systems.
Another advantage of the present invention is that the calibrations of the present technique can be applied to all pulse sequences.
Another advantage of the present invention resides in reduced ghosting.
Still further advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.